Field of the Invention
The present invention relates to an encoding technique for image data.
Description of the Related Art
In recent years, various compression encoding methods for image data have been examined in conjunction with the development of digital image capturing apparatuses such as digital cameras and digital video cameras.
JPEG (Joint Photograph Experts Group) 2000 is given as a representative compression encoding method. In JPEG 2000, an image that is an encoding target is divided into at least one or more tiles. A wavelet transform, quantization, and entropy encoding are performed with a tile as one unit. A lossless 5/3 tap filter, for example, is used in a wavelet transform. In this filter, one high-pass coefficient is generated from three consecutive pixels, and one low-pass coefficient is generated from five consecutive pixels. Consequently, when subjecting pixels at a boundary of the tile to filter processing, pixels outside of the tile are required. A proposal to substitute pixels inside the tile for pixels outside the tile is known (for example, Japanese Patent Laid-Open No. 2007-142615; hereinafter, D1). D1 sets a position of a pixel at a boundary of the tile of interest as a fold position, and uses coefficient data and a pixel in the tile as the coefficient data and the pixel for outside the tile.
However, it is not that a pixel or coefficient data actually present outside of a tile of interest is referred to for the filtering processing. Therefore, a difference occurs between coefficient data after quantization of the tile boundary and coefficient data in a case of referring to a pixel that is actually outside of the tile. This difference means that continuity is lost for tiles in an image obtained by decoding, and consequently noise occurs at tile boundaries.
With respect to this problem, Japanese Patent Laid-Open No. 2004-56213 (hereinafter, D2) discloses a technique of, when encoding a tile of interest, performing encoding processing that includes pixels inside a tile that neighbors the tile of interest. According to D2, in decoding processing of a tile of interest, pixels of the tile of interest and pixels neighboring the boundary with the neighboring tile are obtained. Therefore, a result of removing pixels for the neighboring tile is assumed to be an image that is a decoding result of the tile of interest. According to D2, it is possible to suppress generation of noise at tile boundaries because continuity for adjacent tiles is maintained.
However, although the above described D2 can suppress image degradation, when encoding a tile of interest there is a necessity to perform encoding that includes pixels for the neighboring tile. Therefore, an encoding data amount generated by D2 becomes large in comparison to D1 to a degree that it cannot be ignored. In other words, to cause the amount of encoded data to be equivalent to that of D1 by using the technique of D2, the quantization step in D2 must be larger than that of D1. However, the larger a quantization step is the larger that image degradation becomes.